1. Field of the Invention
This invention relates to compression release brakes for diesel engines.
2. Description of Related Art
Compression-release type brakes are conventionally installed on diesel engines for trucks used on highways. These brakes are used to help slow the vehicles on downhill slopes. Such brakes are operational when the throttle is closed. They operate on the principle of opening the exhaust valve just before top dead center of each compression stroke. The pistons of the engine compress the air within the engine cylinders on the compression stroke, thus slowing the engine and the vehicle. Just before top dead center of the compression stroke, the exhaust valve of each cylinder is cracked open to allow the compressed gases to escape into the exhaust system. Otherwise there would be a rebound effect whereby the compressed gases would act upon each piston during the subsequent expansion stroke, negating the braking effect achieved during the compression stroke.
Compression-release type brakes are usually in the form of a housing fitted between the cylinder head and the cylinder head cover of the engine. Each has a plurality of slave pistons, each slave piston operatively contacting one of the exhaust valves of the engine or, for example, the crosshead for engines having two exhaust valves per cylinder. There is a master cylinder associated with each slave cylinder which conventionally contacts some reciprocating component of the engine which moves at the appropriate time for the associated slave piston. Often this component is an adjustment screw of a rocker arm of the engine. The rocker arm may be, for example, moved by a push tube for a fuel injector of the engine for engines having fuel injectors operated in this manner. Otherwise, the master piston may be powered by the rocker arm for an exhaust valve of a cylinder of the engine other than the one being cracked open.
Conventional diesel engines do not have any component specially timed to power compression-release brakes. Brake manufacturers must design their brakes so they can utilize some component of the engine normally utilized for another purpose, such as the rocker arms mentioned above. Rarely, if ever, is there a component which moves at exactly the right time to crack open each exhaust valve. For example, when rocker arms for other exhaust valves are utilized, they usually begin to move earlier than the manufacturer wants the brake to crack open each exhaust valve. If the valve is cracked too early, then braking effect is lost because the air within the engine cylinder has not been compressed to its maximum extent.
There is another problem in utilizing rocker arms or other valve train components for this purpose. They are designed to open valves or operate fuel injectors and not to power engine brakes. The force required to crack open an exhaust valve just prior to top dead center of the compression stroke is considerable and may place a load on the camshaft or other valve opening components beyond their designed capacity. Accordingly, engine brake manufacturers have made various attempts to utilize rocker arms, or related components, while avoiding the problems that they move too early and then place too high a load on the camshaft and related components.
The fact that the most appropriate rocker arm moves too early to crack open each exhaust valve is usually addressed simply by allowing a gap between the slave piston and the exhaust valve stem so that the initial movement of the rocker arm does not start to crack open the exhaust valve. However, this lost motion wastes potential energy which could be used to open the valve.
The most widely adopted method for utilizing the relatively limited opening abilities of most engine camshafts, particularly their exhaust cams, has been to continuously keep the exhaust valves slightly cracked open during brake operation (typically 0.005"-0.015"). This system has been used in a number of earlier patents, for example U.S. Pat. No. 4,398,510 to Custer and U.S. Pat. No. 4,655,178 to Meneely. By keeping the valves cracked open, less force is necessary to further open the valves against the building cylinder pressure just before top dead center of each compression stroke. This reduces the load on the engine components. However, such systems result in a loss of braking power as a result of air leaving the cylinders through the cracked open exhaust valves on the compression stroke. In addition, they are not easily adapted to many engine brakes which have to be driven by exhaust cams.
Another approach, found for example in U.S. Pat. No. 4,150,640 to Egan or U.S. Pat. No. 4,271,796 to Sickler, is to provide for pressure relief to prevent an overload on the engine. Severe loss of braking power is an undesirable side effect in many cases.
Another approach is an improvement over systems where pressure is relieved, whereby the released fluid is stored both to prevent an overload and then to achieve a more ideal valve opening. U.S. Pat. Nos. 4,706,624 and 4,898,206, both to Meisterick, are examples. Such systems may be somewhat complicated and thus may be expensive or result in reliability problems.
Accordingly, there still remains a need for an engine brake which can overcome timing problems, while avoiding wasteful lost motion and overloading of the normal valve opening mechanism, particularly exhaust cams.